Field of the Invention
The present invention relates to waveguides, and more specifically, it relates to techniques for line selection in such devices.
Description of Related Art
Neodymium lasers typically operate on the four-level transition (4F3/2→4I11/2) at around 1060 nm due to its large cross section and ease of generating a population inversion. However, there is a quasi-three-level transition around 900-940 nm (4F3/2→4I9/2) that is interesting for spectral sensing applications and for its reduced quantum defect. It is challenging to operate a Neodymium-doped fiber laser on the three-level transition around 930 nm. The same issues apply to fiber lasers with other active dopants having multiple transitions.
The challenge for three-level operation of an Nd-doped fiber laser is that the four-level transition achieves inversion much more easily, and typically has a higher gain coefficient. Because of this, the four-level transition will be the first to reach the lasing threshold, after which the gain is clamped, preventing other transitions from reaching threshold.
In a fiber amplifier, the situation is relaxed somewhat because the desired transition is selected by external seeding. However, there will still be competition with the unwanted transition; given the waveguide confinement and high net gain provided by the fiber, ASE on the competing transition can reduce the gain available to the desired one, reduce efficiency and pollute the output. For this reason, active suppression of the unwanted transition is still critical.
One prior art method of accessing the three-level transition relies on careful selection of glass composition to enhance its gain coefficient, along with high intensity pumping to ensure inversion, and bend induced waveguide losses that favor shorter wavelengths. In this case the fiber length is constrained by the requirement that inversion be high over its entire length, with the consequence that efficiency suffers because the pump is incompletely absorbed. Also, while bending does provide wavelength selective loss distributed along the fiber, its selectivity is not great and requires careful adjustment. Finally, for the common double-clad fiber configuration, the need for high pump intensity constrains the clad:core ratio, limiting the prospects for power scaling.
Another prior art method employs a depressed-well core design, which makes a sharp transition from guided to unguided at some wavelength. This design fulfills the need for highly selective distributed loss, but should be considered a short pass edge filter rather than a bandpass filter. Furthermore, the depressed-well design becomes unacceptably tolerance sensitive for large cores, limiting power scaling. Nevertheless, some impressive results have been achieved using it.
A method for active suppression of the unwanted transition uses a hybrid Photonic Crystal/Photonic Bandgap (PCF-PBG) structure for distributed filtering along the fiber. The structures are hybrid in the sense that the modes are confined to the core by a combination of PCF and PBG features. PCF and PBG structures support versatile and robust spectral features and provide an extensive design space for the modal and spectral properties of optical fibers.